Instances in which the system is successful if at least one of n
parallel paths is successful has been discussed. In other instances, at least
k out of n elements must be successful. In such cases, the reliability of the
redundant group (each with the same Probability of Success, p) is given by a
series of additive binomial terms in the form of

Use of the binomial formula becomes impractical for hand
calculation in multi-element partial redundant configurations when the values
of n and k become large.^{4} In these cases, the normal
approximation to the binomial may be used. The approach can be best
illustrated by an example.

__Example 2:__

A new transmitting array is to be designed using 1000 RF
elements to achieve design goal performance for power output and beam width. A
design margin has been provided, however, to permit a 10% loss of RF elements
before system performance becomes degraded below the acceptable minimum level.
Each element is known to have a failure rate of 1000 x
10^{-6} failures per hour. The proposed design is illustrated
in Figure 7.5-11, where the total number of elements is n = 1000; the number
of elements required for system success is k = 900; and, the number of element
failures permitted is r = 100. It is desired to compute and plot the
reliability function for the array.